[Contact] 2-tube TX

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Re: [Contact] 2-tube TX

Post by Admin on Wed May 14, 2014 5:56 pm

Hi Peter,
The 1nf capacitor is in effect a (relatively) dead-short to RF and it can be ignored when it comes to matching.

The object of the transmitter was to be simplicity itself: one band switch and one tuning control. A PI filter would require the usual back-and-forth tuning but a single tuning control + band switch is simplicity in itself. If you also look at the maths then the input capacitor for such a high impedance is almost nothing.

To use a PI filter would also require two variable capacitors, which to be quite honest, I did not have around at the time. Today the air-spaced tuning cap is about as common as a truthful politician.

Besides that, with a traditional two caps and one inductor one would still need to have the isolating cap and the safety inductor to ground. Air-spaced tuning caps used to suffer from "schoolboy vanes" where these nice parallel vanes look "interesting" and it takes but little effort to cause them to become short circuit. So a configuration relying on the variable caps for DC isolation is also dangerous. The last thing one would want is to electrocute a neighbour. ... . on second though perhaps it could be a good thing - you do not know my neighbour; Pelle Plutt, and his wife Cilly Plutt.

But you have a nice day. I hope that you like the project. I had fun designing it and the low power level using tubes/valves makes impedance matching simple with just the two components.

Very best regards (med vänlig hälsning) from harry - SM0VPO

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Re: [Contact] 2-tube TX

Post by pirx on Tue May 13, 2014 10:24 pm

OK, I understand. But why not add a capacitor to form a pi filter? With the L filter you can match a low impedance antenna which is ok for a short wire. pi filter gives more freedom. Or the capacitor could be switchable from the present position to the output so high impedance antennas could be matched.
Take care,
Peter

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[Contact] 2-tube TX

Post by Admin on Sun May 11, 2014 5:50 pm

[Contact] 2-tube TX
<I will translate to English below>


From Guest To Admin, Today at 13:49
Hej!
Massor med förnämliga projekt på din hemsida!
Jag tittade på tvårörssändaren som påminner mig om min egen som jag byggde för 40 år sedan...
Fast jag hade EF80 i oscillatorn och EL84 som final.
Skall du underlätta för den nya generationen finns det utrymme för små förbättringar: Resistorn i grid2 samt katodkondensatorn på 6AK5:an är omärkta.
Sedan en fråga: jag inser inte vad har 2.2 mH drossel parallellt med utgången för funktion.
I min rig hade jag en kondensator där så att det blev ett pi-filter. Tacksam för förklaring,
73 de Peter
 
Hej Peter
 
> Massor med förnämliga projekt på din hemsida!
Toppen!! Så glad att du tycker om de.
 
> Jag tittade på tvårörssändaren som påminner mig  ...
Det var hela meningen och hoppas du kan dela dina idea och projekt med oss.
 
> Sedan en fråga: jag inser inte vad har 2.2 mH drossel parallellt med utgången för funktion.
Det är vanligt praxis att ha en DC-short (likström kortslut) till jord på antenna kontakter. Om koppling kondensatorn ska läcka lite DC då finns det risk för 100-tals volts på antennen - hela antenn systemet kan vara livsfarligt. 2,2mH har 0,3mm Dia. koppar som kan leda flera ampere för en kort stund: de räcker för att blåsa proppen (eller halvleradre likriktare).
 
Hoppas förklaringen är ok för dig? Om di ha flera frågor so det är bara att skriva. Tyvärr är jag upptagen p.g.a. att flytta så jag kanske inte kan svara dina frågor på en gång. Jag blir tacksam om du kan använder forum då kan andra ger ett svar.
 
Mvh Harry

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Hello!
Lots of prestigious projects on your website!

I looked at two tube transmitter, which reminds me of my own that I built 40 years ago ...
Though I had EF80 in the oscillator and EL84 as final.

Will you make it easier for the new generation, there is room for small improvements: Resistor in grid2 and the cathode capacitor on 6AK5: an unmarked (just a BIG value > 1nf).

Then a question: I do not realize what has 2.2 mH inductor in parallel with the output of the function.
In my rig I had a condenser , where it became a pi filter. Grateful for explanation ,
73 de Peter
 
Hi Peter
 
> Lots of prestigious projects on your website!
The top ! So glad you enjoy those .
 
> I looked at two tube transmitter, which reminds me ...
That was the whole sentence and hope you can share your ideals and projects with us.
 
> Then a question: I do not understand what has the 2.2 mH inductor in parallel with the output for function?
It is common practice to have a DC-short (kortslutning) to ground the antenna connections. If the (output) coupling capacitor should leak DC then there is a risk of 100s of volts on the antenna - the whole antenna system can be fatal. 2.2 mH has 0.3 mm Dia. copper that can lead many amps for a short while, which will be sufficient to blow the fuse (or semiconductor rectifiers).
 
Hope the explanation is ok for you? If you have more questions then it is just to write. Sorry, I'm busy because of moving house so I may not respond your questions at once. I will be grateful if you could then use the forum then others the chance to give an answer.
 
Regards Harry

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